1. Field of the Invention
The present invention relates to a data-transmitting method for wireless sensor network and, particularly, to a constructing method creating a data transmission model by a hierarchical data clustering technique to lower power consumption of a wireless sensor network.
2. Description of the Related Art
Nowadays, wireless sensor networks applied in environmental observation are implemented by disposing a plurality of sensors with low cost and small volume in a predetermined area and transmitting data between said sensors. For example, when a wireless sensor network is constructed in a hothouse, factors that affect the growth of plants such as temperature, humidity, illumination and sounds may be effectively detected. How to lower power consumption and cost of construction and to raise stability of data transmission are the critical points for the implement of the wireless sensor network. However, once the wireless sensor network is applied, every change in position of a mobile sink collecting data through sensors of a wireless sensor network causes the routes for data transmission to be rebuilt, which may easily induce a global flooding. Thus, an extra power consumption may occur.
In order to solve the above-mentioned problem about power consumption, several conventional data-transmitting methods realized by wireless sensor networks with grid-based data transmission models are provided. For example, Haiyun Luo et al. proposed a method named “two-tier data dissemination (TTDD)” for the same reason. Referring to FIG. 1, for carrying out the TTDD method, a wireless sensor network 1 having a plurality of nodes 11 and at least one sink 12 is pre-built in an area, with the area being divided into a plurality of virtual cubes 13 that jointly construct a matrix. Furthermore, several nodes 11 adjacent to the intersections of boundaries of the virtual cubes 13 are identified as dissemination nodes 111. Besides, it is supposed that each node used in the TTDD method is aware of other nodes' positions.
For different levels of data transmission, the TTDD method provides a two-staged transmitting method: a “lower-tier query grid forwarding” stage and a “higher-tier query grid forwarding” stage. In detail, within a virtual cube 13, the data transmission is done through the lower-tier query grid forwarding stage; and if the data transmission has to be done beyond a single virtual cube 13, the higher-tier query grid forwarding stage is then executed.
In the lower-tier query grid forwarding stage, when a sink 12 raises a quest for data collection, a closest node 11 to the sink 12 will be identified as a primary agent (PA), and then one of the closest dissemination nodes 111 to the PA will handle the further data transmission. Now referring to FIG. 2, if the position of the sink 12 is changed and is out of a transmission range of the PA, another node 11 which is presently the closest node 11 to the sink 12 will then be identified as an immediately agent (IA). Then, the IA will continuously transmit quests for data collection from the sink 12 to the PA without breaking off the data transmission.
Referring to FIG. 1 again, in high-tier query grid forwarding, the data transmission over several virtual cubes 13 is carried out by transmitting data along a link between two of the dissemination nodes 111. Therefore, when a flooding exists about the sink 12, instead of diffusing all over the wireless sensor network 1, the flooding will be restricted merely within the virtual cube 13 where the sink 12 is.
However, although the conventional TTDD method can restrict the flooding within a virtual cube 13 containing the sink 12 to improve power saving, data transmission within the virtual cube 13 still raises a large amount of wasted power. Thus, the TTDD method may still cause unbalanced power consumption over the wireless sensor network 1. Hence, there is a need of improving the conventional clustering techniques.